Anomalous Coulomb Drag in Electron-Hole Bilayers due to the Formation of Excitons

TL;DR

This paper provides a theoretical explanation for the anomalous temperature dependence of Coulomb drag in electron-hole bilayers, attributing it to exciton formation and ionization-recombination dynamics, aligning with experimental data.

Contribution

It introduces a model considering exciton formation and ionization-recombination crossover to explain Coulomb drag behavior, differing from electron-hole pairing theories.

Findings

Drag resistivity shows a minimum and sharp upturn at low temperatures.

Resistivity is insensitive to electron-hole concentration mismatch.

Qualitative agreement with experimental temperature dependence.

Abstract

Several recent experiments have reported an anomalous temperature dependence of the Coulomb drag effect in electron-hole bilayers. Motivated by these puzzling data, we study theoretically a low-density electron-hole bilayer, where electrons and holes avoid quantum degeneracy by forming excitonic molecules. We describe the ionization-recombination crossover between the electron-hole plasma and exciton gas and calculate both the intralayer and drag resistivity as a function of temperature. The latter exhibits a minimum followed by a sharp upturn at low temperatures in a qualitative agreement with the experimental observations [see, e.g., J. A. Seamons et al., Phys. Rev. Lett. 102, 026804 (2009)]. Importantly, the drag resistivity in the proposed scenario is found to be rather insensitive to a mismatch in electron and hole concentrations in sharp contrast to the scenario of electron-hole Cooper pairing.